Proximity-based applications and services represent an emerging social-technological trend. In 3rd Generation Partnership Project (3GPP), a Proximity Service (ProSe) capability is to be introduced in Long Term Evolution (LTE), in particular in specification Release 12 (Rel-12). ProSe is also known as Device-to-Device (D2D) communication and enables direct communication for instance between nearby mobile terminals of a wireless system.
D2D communication has been proposed as an underlay to cellular networks, i.e. D2D using the same spectral resources as used for the cellular network. This would take advantage of the proximity of communicating devices and at the same time allow devices to operate in a controlled interference environment. The D2D communication could share the same spectrum as the cellular communication for example by reserving some of the cellular uplink resources for D2D purposes. Allocating dedicated frequency spectrum for D2D purposes is a less attractive alternative since spectrum is a scarce resource. A dynamic sharing of resources between the D2D services and cellular services is more flexible and provides higher spectrum efficiency.
In Rel-12 there is only a forward link (from the sender to the receiver) for D2D communication and no reverse link (from the receiver to the sender). In Rel-13, D2D reverse link may be introduced. In such reverse link, the reception node may provide channel state information (CSI) feedback, radio resource management (RRM) measurements (for instance Reference Signal Received Power, RSRP, or Reference Signal Received Quality, RSRQ), and acknowledgements (ACKs) and negative acknowledgments (NACKs). For either forward link or reverse link, power control may be employed for improving the D2D link performance and efficiently reduce the interference towards the wireless access network (WAN) system (e.g. LTE).
According to current LTE specification for D2D transmission, the D2D transmission power in a D2D side link is subject to cellular power control. With the cellular power control, the transmission power in D2D side link may be changed, which will lead to the transmit power of the reference signal in the D2D link changing dynamically. The transmission power is subject to dynamic changes for other reasons as well, e.g. due to one or both of the communicating parties moving fast and/or moving into a less favorable radio environment.
With such dynamically changing transmission power it is a challenge for the reception node to estimate the pathloss to the transmitting node, and thus difficult to perform power control for the D2D reverse link in order to mitigate D2D link interference. Further, it is also a challenge for the reception node to estimate CSI for the D2D forward link due to the dynamically changing transmission power. There is a need for methods to handle these and other problems relating to the D2D link resulting e.g. from the dynamically changing transmission power.